Modification of polyethylene resins in the melt state using free radical initiators is known. One such process for the production of polyethylene films is described in U.S. Pat. No. 5,962,598. The process involves melt extruding a linear copolymer at a temperature of at least 180° C. in the presence of an amount of free radical initiator sufficient to improve bubble stability. Melt phase processes of this type are, however, difficult to control and often produce undesirable amounts of gels.
Free radical initiators, particularly, peroxides have also been utilized for the solid state oxidation of polyethylene resins to produce emulsifiable products. Such processes are described in U.S. Pat. Nos. 3,322,711; 4,459,388; 4,889,897; 5,064,908 and 5,401,811. Whereas these reactions utilize free radical initiators with particulate polyethylene resins, they are typically carried out under conditions so that polyethylenes having high acid numbers are produced.
In one process for the oxidation of polyethylene in a fluidized bed reactor (U.S. Pat. No. 4,459,388) high density polyethylene powder is combined with 500 to 20000 ppm free radical forming compound in the absence of oxygen and heated below the softening point of the high density polyethylene to decompose the initiator prior to introduction into a fluidized bed reactor where it is oxidized. Treatment of polyethylenes utilizing high levels of free radical initiators in this manner produces low molecular weight species which limits utility to certain applications, namely, water-emulsifiable applications.
In copending application Ser. No. 10/930,295, multimodal polyethylene powders obtained by blending high and low molecular weight components made in multistage polymerization reactors are modified in the solid state utilizing free radical initiators at levels from 1 to 4500 ppm. Multimodal polyethylene resins modified in accordance with this process have reduced die swell and increased melt strength.
It would be highly advantageous if other polyethylene resins could be modified with low levels of peroxide in the solid state to enhance one or more of their melt characteristics.